Spectrophotometers—such as ultraviolet and visible spectrophotometers and atomic absorption spectrophotometers—use spectrometers (monochromators) to obtain monochromatic light of a predetermined wavelength. The typical configuration of a spectrometer is to include a diffraction grating (wavelength dispersion device) and a rotational driving mechanism for changing the angle of light that is incident to the wavelength dispersion device. With a spectrometer such as the afore-described, light that is chromatically dispersed by the diffraction grating passes through an exit slit whose position is fixed to extract monochromatic light of a predetermined wavelength. The wavelength of the monochromatic light that is extracted is changed by rotating the diffraction grating by means of the rotational driving mechanism. Therefore, the accuracy of the wavelength of the monochromatic light is strongly dependent on the angular accuracy of the diffraction grating. Increasing the accuracy of the wavelength requires a rotational driving mechanism that can rotationally drive the diffraction grating with a very fine pitch and accuracy.
Previously known in the art is a rotational driving mechanism that uses a sine bar mechanism. Others have proposed rotational driving mechanisms that use a wave gear device which is a type of reduction gear (see for example Patent Literature 1).
As FIG. 5 shows, a wave gear device—also known as a harmonic drive (registered trademark)—comprises a wave generator 71 which is a cam-shaped oval component that is coupled to an input shaft 75, a cup-shaped elastic gear (flex spline) 73 that is installed on the outer periphery of the wave generator via bearings 7, and a cylindrical rigid gear (circular spline) 74.
Rigid gear 74 has a circular cross-sectional shape, and the elastic gear 73 is flexed into an oval cross-section by the wave generator 71. This results in the gear teeth on the inner peripheral surface of the rigid gear 74 and the gear teeth on the outer peripheral surface of the elastic gear 73 to mesh with each other at only two locations located close to the major axis of the oval-shaped wave generator 71 and to be completely separated from each other near the minor axis.
If the rotation of a motor is transmitted to the wave generator 71 via the input shaft 75 while the rigid gear 74 is fixed, the elastic gear 73 elastically deforms with the rotation of the wave generator 71 and the meshing position with the rigid gear 74 sequentially moves. The number of gear teeth that is disposed on the elastic gear 73 is less by 2 than the number of gear teeth that is disposed on the rigid gear 74. This means that during one rotation of the wave generator 71, the elastic gear 73 rotates in the opposite direction by two gear teeth. The rotation of the elastic gear 73 is extracted as an output and is transmitted to the diffraction grating.
With a wave gear device, the common practice is to fix the rigid gear 74 as afore-described and to extract the rotation of the elastic gear 73 as an output as afore-described. It is also possible to fix the elastic gear 73 and extract the rotation of the rigid gear 74 as an output.